如何在MySQL的内部联接中使用WHERE IN

Question

I have 3 tables(subjects, student and exam) I'm joining to fetch a student result based on class assigned to a particular teacher.

Let's say a teacher is being assigned two classes(SS 2 and SS 3) and from the exam table I only want to select students result based on the class assigned to that teacher.

Here are the tables structures

    CREATE TABLE `student` (
      `id` int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
      `regiNo` varchar(100) NOT NULL,
      `session` varchar(15) NOT NULL,
      `class` varchar(50) NOT NULL,
      `class_group` varchar(26) NOT NULL,
      `firstName` varchar(60) NOT NULL,
      `middleName` varchar(60) NOT NULL,
      `lastName` varchar(60) NOT NULL

     )

INSERT INTO `student` (`id`, `regiNo`, `session`, `class`, `class_group`,  `firstName`, `middleName`, `lastName`) VALUES
(8, 'TS2018007',  '2018', 'Primary One', 'Science',  'ADEWUYI', 'o', 'ODERINDE'),
(9, 'TS2018009',  '2018', 'Primary One', 'N/A',  'Ayodele', 'Jumide', 'Boros')

     CREATE TABLE `exam` (
      `examid` int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
      `Roll_Number` varchar(30) NOT NULL,
      `sub_id` varchar(20) NOT NULL,
      `mid_term` varchar(5) NOT NULL DEFAULT '',
      `count_assessment` varchar(4) NOT NULL DEFAULT '',
      `scored` varchar(30) NOT NULL,
      `total_score` varchar(4) NOT NULL DEFAULT '',
      `sub_highest` varchar(4) NOT NULL DEFAULT '',
      `sub_lowest` varchar(4) NOT NULL DEFAULT '',
      `first_term` varchar(4) NOT NULL DEFAULT '',
      `second_term` varchar(4) NOT NULL DEFAULT '',
      `third_term` varchar(4) NOT NULL DEFAULT '',
      `avg_score` varchar(4) NOT NULL DEFAULT '',
      `internaltype` varchar(30) NOT NULL,
      `Class` varchar(40) NOT NULL,
      `Year` varchar(4) NOT NULL
    )

INSERT INTO `exam` (`examid`, `Roll_Number`, `sub_id`, `mid_term`, `count_assessment`, `scored`, `total_score`, `sub_highest`, `sub_lowest`, `first_term`, `second_term`, `third_term`, `avg_score`, `internaltype`, `Class`, `Year`) VALUES
(32, 'TS2018007', 'Eng', '54', '20', '50', '134', '80', '70', '45', '12', '43', '45', 'First', 'J.S.S 3', '2019'),
(33, 'TS2018007', 'Agr', '32', '32', '12', '34', '56', '23', '34', '12', '56', '20', 'First', 'J.S.S 3', '2019'),
(34, 'TS2018007', 'math', '54', '20', '45', '12', '45', '34', '34', '12', '12', '23', 'First', 'J.S.S 3', '2019'),
(35, 'TS2018007', 'bio', '54', '27', '23', '23', '12', '12', '45', '55', '12', '45', 'First', 'J.S.S 2', '2019');


    CREATE TABLE `subjects` (
      `id` int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
      `subect_code` varchar(55) NOT NULL,
      `subjects` varchar(255) NOT NULL
    )

INSERT INTO `subjects` (`id`, `subect_code`, `subjects`) VALUES
(12, 'Eng', 'English Language'),
(13, 'Agr', 'Agricultural Science'),
(17, 'math', 'Mathematics'),
(20, 'bio', 'Biology'),
(21, 'fur', 'Further Maths'),
(25, 'che', 'Chemistry'),
(26, 'geo', 'Geography');

How can I select data from MySQL using WHERE IN in inner join in MySQL query? The query below returned empty result set:

SELECT distinct s.regiNo, s.firstName as fname, s.middleName as mname, s.lastName as lname, s.gender, s.class_group, c.subjects,
e.mid_term, e.count_assessment, e.examid, e.scored, e.internaltype, e.Class, e.Year,e.total_score,e.sub_highest,e.sub_lowest,e
.first_term,e.second_term,e.third_term,e.avg_score FROM student s inner join exam e on s.regiNo = e.Roll_Number 
inner join subjects c on e.sub_id = c.subect_code WHERE e.Class IN('S.S 2', 'S.S 3')

Answer 1

To debug this:

• remove the joins, remove joined rows from select, and confirm results
• add one join, make it a left join , and confirm results
• change left join to inner join and confirm results
• Do the previous 2 steps with your other join.

The inner joins can end up excluding all rows, especially with 2 of them.

Answer 2

Thanks, guys. I discovered I was the one making mistakes. The specified values in my IN clause did not match any value in the table row which was the reason for it returning null values.

SELECT DISTINCT s.regiNo, 
s.firstName as fname, 
s.middleName as mname, 
s.lastName as lname, 
s.gender, 
s.class_group, 
c.subjects, 
e.mid_term, 
e.count_assessment, 
e.examid, 
e.scored, 
e.internaltype, 
e.Class, 
e.Year,
e.total_score,
e.sub_highest,
e.sub_lowest,
e .first_term,
e.second_term,
e.third_term,
e.avg_score 
FROM student s 
INNER JOIN exam e 
ON s.regiNo = e.Roll_Number 
INNER JOIN subjects c ON 
e.sub_id = c.subect_code 
WHERE e.Class IN('J.S.S 3','J.S.S 2') 
GROUP BY
s.regiNo, 
s.firstName, 
s.middleName, 
s.lastName, 
s.gender, 
s.class_group, 
c.subjects, 
e.mid_term, 
e.count_assessment, 
e.examid, 
e.scored, 
e.internaltype, 
e.Class, 
e.Year,
e.total_score,
e.sub_highest,
e.sub_lowest, 
e.first_term,
e.second_term,
e.third_term,
e.avg_score